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UNIT III FIBER OPTICAL SOURCES AND COUPLING. Direct and indirect Band gap materials- LED LED structures Light source materials Quantum efficiency and LED power, Modulation of a LED, Lasers Diodes Modes and Threshold condition Rate equations External Quantum efficiency
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Direct and indirect Band gap materials- • LED • LED structures • Light source materials • Quantum efficiency and LED power, • Modulation of a LED, • Lasers Diodes • Modes and Threshold condition • Rate equations • External Quantum efficiency • Resonant frequencies • Temperature effects, • Introduction to Quantum laser • Fiber amplifiers • Power Launching and coupling, • Lencing schemes, • Fiber -to- Fiber joints, • Fiber splicing • Signal to Noise ratio , • Detector response time.
Characteristics of optical Sources • The wavelength of the light emitted by these sources must lie within the low loss windows of optical communication • The optical source should have a narrow spectral width. (The dispersion caused in an optical fiber is directly proportional to the spectral width of the source and so, to have low dispersion, the optical source should have very narrow spectral widths). • The optical source should be capable of coupling enough optical energy into the optical fiber
Coupling of the source of light to the optical fiber must be possible with great ease. • The optical source should possess high modulating speeds that correspond to optical frequencies • The source of light should be highly reliable.
Semiconductor sources • light emitting diodes (LEDs) • Injection LASER diodes (ILDs)
A semiconductor material is categorized on the basis of the energy band structure of the semiconductor. • The energy band of a semiconductor material consists of three distinct energy bands, • conduction band, • valence band and • the forbidden band
conduction band • the conduction band has free electrons • which have energies greater than Ec, the lowest energy level of the conduction band. • An equal number of positively charged holes are present in the valence band of an intrinsic semiconductor material • which have energies smaller than Ev, the highest energy level of the valence band.
The energy difference between the conduction band and the valence band is called the forbidden band. • The forbidden band is called the band-gap of the semiconductor material and is given by:
The recombination of an electron and a hole take place with the release of energy which is equal band Gap Energy. • The released energy is given out in the form of radiation. • If the frequency of the radiation falls within the visible range, a photon is said to be emitted and the process is known as Radiative Recombination. • The energy of the emitted photon is equal to the product of the frequency of the radiation and the Planck’s constant (6.626x10-34 Js). • If the recombination does not result in the emission of a photon, the process is known as Non-Radiative Recombination
A semiconductor material is classified into two types on the basis of its energy band diagram in the energy-momentum space. • Direct band-gap semiconductor materials. • Indirect band-gap semiconductor materials.
The schematic representation of the energy-band diagrams of these two types of semiconductors is shown in figure
In direct band-gap (DBG) semiconductor maximum energy level of the valence band Coincides with the minimum energy level of the conduction band with respect to momentum . • In a DBG semiconductor, a direct recombination takes place with the release of the energy equal to the energy difference between the recombining particles.
In Indirect band-gap (IBG) semiconductor the maximum energy level of the valence band does not coincide with the minimum energy level of the conduction band with respect to momentum. • IBG semiconductor, due to a relative difference in the momentum, first, the momentum is conserved by release of energy and only after the both the momenta coincide themselves, a recombination occurs with the release of energy. • The probability of a radiative recombination, hence, in case of IBG semiconductor is much less to that in case of DBG semiconductors. • Hence, the efficiency factor of a DBG semiconductor is much more than that of a IBG semiconductor.
That is the reason why DBG semiconductors are always preferred over IBG for making optical sources. • The two well-known intrinsic semiconductors, Silicon and Germanium are both IBG semiconductors , hence cannot be used to manufacture optical sources. • DBG semiconductor material used is Gallium Arsenide (GaAs). • It has a high efficiency factor which means that a large portion of the recombinations in GaAs are radiative in nature. • Apart from GaAs, there are also other materials which have direct band-gap nature. • Most of them are the alloys of type III and type IV elements of the periodic table.
Under the effect of the forward bias voltage, the excess carriers are repelled towards the depletion region where they recombine and emit the recombination energy in the form of radiation
LED Structures: Homojunctions and Heterojunctions • A simple LED may be constructed by forming a pn junction using one of the direct band gap semiconductors. • Such an LED is called a homojunction LED as the pn junction is made the same material. • An LED for use in optical fiber communications must have > high radiance output, >fast emission response time >high internal quantum efficiency.
Homojunction LEDs are not good from the point of view of high radiance and high quantum efficiency. • In order to have a device with high radiance and high quantum efficiency the LED structure must provide carrier confinement and optical confinement. • Carrier confinement would improve the radiance and internal quantum efficiency. • Emitted photons can get absorbed by the surrounding materials, which can be prevented by better optical confinement.
Heterojunctions • Carrier and optical confinement can be achieved by the use of single and double heterojunctions instead of homojunctions. • In a heterojunctionpn device the junction is formed out of dissimilar crystalline semiconductors. • These materials have unequal band gaps. • A combination of multiple heterojunctions together in a device is called a heterostructure. • It is possible to have both single heterostructures and double heterostructures. • In a heterostructure LED the device has an active region, catering to radiative recombination, sandwiched between two different alloy layers.
These devices achieve carrier confinement through differences in the band gaps of the adjacent layers. • Also, the active region material is chosen such that its refractive index is higher than the adjacent materials. • Because of these differences in the refractive indices the LED structure acts like a slab waveguide with the active region as the central (core) region. This achieves optical confinement of the emitted photons.
LED Configurations for Fiber optics • The LED structures can be classified as > surface-emitting LED(SLED) > Edge emitting LED (ELED) • Depending on whether the LED emits light from a surface that is parallel to the junction plane or from the edge of the junction region. • Both types can be made using either a pnhomounction or a heterostructure design in which the active region is surrounded by p- and n-type cladding layers.
Surface Emitting LED • In a surface emitting LED the plane of the active light-emitting region is oriented perpendicular to the axis of the fiber. • The active area is typically circular, roughly 50 μm diameter and upto 2.5 μm thick . • The active region is made narrow to aid in carrier confinement. • A well is etched through the substrate of the device and an optical fiber is cemented to the well to accept the emitted light. • The emission pattern of this type of LED is isotropic with a 120 degrees half-power beam width. • Such an emission pattern is called a lambertian pattern. • In this pattern, the source looks equally bright when viewed from any direction, but the optical power decreases as cos θ, where θ is the angle between the viewing direction and the normal to the surface.
Edge Emitting LED • The structure of an edge emitting LED consists of an active junction region and two guiding regions. • The refractive index of the guiding layers is lower than that of the active region, but higher than the index of the surrounding material. • The active area 50-100 μm diameter. • This type of a structure forms a waveguide channel for the optical radiation. • The beam emitting from an edge emitting LED is more directional than a surface emitter. • The beam is generally elliptical. • Due to waveguiding effect the beam is narrower in the plane perpendicular to the pn junction - the full width at half maximum (FWHM) is typically 25 to 35 degrees. • However, since there is no waveguiding in the lateral direction, i.e. in the plane parallel to the junction, the beam in this plane is lambertian with a FWHM of 120 degrees.
Internal Quantum Efficiency • The internal quantum efficiency ηint is an important parameter of an LED. • It is defined as the fraction of the electron-hole pairs that recombine radiatively. • If the radiative recombination rate is Rr and the non-radiative recombination rate isRnr, • then the internal quantum efficiency is the ratio is the ratio of the radaitive recombination rate to the total recombination rate. • ηint is typically 50% in homojunction LEDs, but ranges from 60 to 80% in double-heterostructure LEDs.
Optical Power • If the current injected into the LED is I, then the total number of recombinations per second is I/q.(whereq is the electron charge). • Total number of radaiative recombinations is equal to (ηint I/q). • Since each photon has an energy hν, the optical power generated internally by the LED is: Pint = (ηint I/q)(hν).
External Quantum Efficiency • The external quantum efficiency (ηext)of a LED is defined as the ratio of the photons emitted from the LED to the number of internally generated photons. • Due to reflection effects at the surface of the LED typical values of ηout are < 100%.
